There is now increasing use of epicyclic change-speed transmissions in heavier vehicles such as trucks and long distance buses since epicyclic gears are capable of transmitting high torques and also lend themselves to automatic control, thereby reducing driver fatigue. The incorporating of a torque converter, with its capability of torque multiplication, in the input to the transmission enables the latter to have fairly widely spaced ratios since the losses in the torque converter when acting as a torque multiplier are found to be acceptable for short periods, for example when accelerating the vehicle from rest. When the vehicle has attained its cruising speed, the transmission will normally be in direct drive (when en bloc rotation of its epicyclic gearing) and the torque converter will be acting as a two-element fluid coupling (i.e. its coupling state, without torque multiplication) or may be locked-up by means of a lock-up clutch.
A problem then arises when the vehicle encounters an uphill gradient sufficient to slow the vehicle down below its cruising speed and into the torque-multiplication range of the torque converter if the lock-up clutch of the latter is disengaged. The transmission becomes less efficient with a corresponding reduction in available power. If the next lower ratio is engaged, to enable the torque converter to operate in its coupling range, the vehicle must be appreciably slowed down to avoid overspeeding the engine in view of the large step between this ratio and direct drive.
Thus, an object of the present invention is to provide an epicyclic vehicle transmission in which the lower ratios are relatively widely spaced while the ratios nearer to unity are relatively closely spaced while at the same time avoiding the use of a separate splitter in series with the main unit, in view of the difficulties in obtaining synchronisation of operation of two such units in series.